<!DOCTYPE html>
<html>
<head>
    <meta charset="UTF-8">
</head>
<body>
    <style>
    .spacer {
      height: 100vh;
    }
    
    .sticky-footer {
        position: sticky;
        bottom: 0;
        background: #fff;
        border-top: 2px solid #e9ecef;
        padding: 1.5rem 0.25rem;
        background-color: #2c59859c;
    }
    </style>
    <form>
        <section>
          <header class="spacer">Section A</header>
          <br><br>
          <div class="field">
            <div id="input"><label>Input</label> <input type="text"></div>
          </div>
        </section>

        <footer class="sticky-footer">
          <button> save </button>
        </footer>
    </form>
</body>
</html>